Proteins are highly complex polymers comprised of amino acid chains connected by peptide linkages and occur in the cells of all living organisms. Protein comprises the basic component of connective tissue in the form of collagen, and is also the basic component of hair, nails, feathers and skin in the form of keratin. Protein is a term that generally describes complex nitrogenous substances, commonly known as gelatin, fibrin and casein.
Animal gelatin is a commercially important proteinaceous material obtained by the partial hydrolysis of collagen, which is the chief protein component in skins, bones, hides, and white connective tissues of the animal body. Animal glue and proteolytic enzyme-hydrolyzed gelatin are lower molecular weight protein products obtained by a more complete hydrolysis of collagen, and thus can be considered as containing lower molecular weight fractions of gelatin.
Animal glue and animal gelatin are almost identical. They are produced by the same methods and from the same raw materials but differ in purity and quality. In some instances, however, animal glue is referred to as technical gelatin. Gelatin is used in the food, pharmaceutical and photographic industries that take advantage of its gelatin properties which include: a reversible gel-to-sol transition in aqueous solution; a capability to act as a protective colloid; water permeability; insolubility in cold water but complete solubility in hot water resulting in viscous warm aqueous solutions.
For commercial applications, a water-soluble gelatin is generally desired. Aqueous solutions of gelatins are used in candies, dairy products, meat products, and primarily in gelatin desserts. In some uses, a gelatin composition is in the form of a relatively large gelled block, as in a dessert; while for other uses, it is in the form of an extremely thin, gelled film, as in a photographic subing, and in the form of a liquid glue in an adhesive.
The viscosity of aqueous gelatin solutions generally increases with increasing gelatin concentration and decreasing temperature. In some applications, such as photographic film products, the gelatin matrix is cross-linked by chemical means to provide a hardening that permanently changes the solubility of the gelatin.
In most edible gelatin products, a Type A gelatin is used. Type A gelatin is produced by an acid hydrolysis curing process of collagenous raw materials using techniques that are well-known in the art. Type A gelatin has an isoelectric point between a pH value of about 7 and 9. Some Type B gelatin is also used in edible gelatin products. Type B gelatin is produced by an alkali hydrolysis curing process, usually a liming process, that is well-known in the art. Type B gelatin has an isoelectric point between a pH value of about 4.6 and about 5.2.
A cross-linked gelatin is desirable for pharmaceutical enteric capsules. The gelatin used for soft capsules is generally either a Type A gelatin having a low-Bloom value of about 170 to about 180 grams, a Type B having a low-Bloom value of about 150 to about 175 grams, or a mixture of Types A and B. Hard capsules are generally made of a Type A gelatin having a medium-to-high Bloom value of about 250 to about 280 grams, a Type B having a medium-to-high Bloom value of about 225 to about 250 grams or combinations thereof.
Bloom value or Bloom gel strength is well-known in the art as a property of gelatin which relates to the gel strength or firmness which a particular gelatin composition produces under standardized conditions. The Bloom gel strength or Bloom value is based upon the force in grams required to cause a standardized piston to move through 4 millimeters (mm) of a standardized gelled sample, and is measured at a temperature of 10 degrees C. (about 50 degrees F.) with a commercially available machine commonly known as a Bloom Gelometer. A higher Bloom value indicates a higher gel strength and a higher gelling temperature than that of a lower Bloom value for a selected amount of gelatin.
Gelatins and glues are generally graded on the basis of Bloom gel strength, determined as hereinbefore described, and by the viscosity of an aqueous solution. Viscosity is usually measured as a flow time in millipoises at about 60 degrees C. (about 140 degrees F.) through a standard pipette or in a U-tube viscometer using procedures that are well-known in the art. Thus, the Bloom gel strength and viscosity of both these proteins plays an important part in their commercial utility.
There is a need for a cross-linked gelatin composition having improved Bloom gel strength and increased viscosity, where the gelatin composition has been cross-linked with a material that is generally recognized as safe (GRAS) by the United States Food and Drug Administration for use in edible products and pharmaceuticals. Persons skilled in the art will appreciate that a desirable cross-linked gelatin composition for use in edible products and pharmaceuticals should be clear and substantially color-free. It is to be understood that the term "substantially color-free" as used herein with reference to a cross-linked gelatin composition means a composition that retains the straw color inherent in food grade gelatin. There is also a need in industrial applications for a clear substantially color-free gelatin that retains its Bloom gel strength and its viscosity over a relatively wide range of useful pH values.